Organic thin film solar cells are expected to be used as future low-cost solar cells since they can be produced easier and with lower equipment costs than conventional silicon and compound semiconductor solar cells.
In organic thin film solar cells of the p-n heterojunction type using p-type and n-type organic semiconductors, the organic semiconductors form excitons in which electron-hole pairs are strongly bound, and these diffuse and migrate to the interface of p-n junctions. Excitons are charge-separated into electrons and holes due the strong electric field at the interface, and the resulting electrons and holes are transported to respectively different electrodes resulting in the generation of electromotive force. However, in solar cells employing this configuration, since the diffusion length of the excitons is short at roughly only several tens of nanometers, carrier paths are actually able to be effectively formed only within a distance range of several tens of nanometers from the p-n junction interfaces, thus resulting in an extremely low conversion efficiency.
As a result of subsequent progress in the area of bulk heterojunction technology, enabling p-type organic semiconductors (donors) and n-type organic semiconductors (acceptors) to be combined so that p-n junctions can be dispersed within a thin film at the nanometer level, the conversion efficiency of organic thin film solar cells improved considerably. For example, J. Xue, S. Uchida, B. P. Land and S. R. Forrest et al. describe an organic thin film solar cell having a bulk heterojunction configuration as shown in FIG. 9 (Appl. Phys. Lett., 85, p. 5757 (2004)). A photoelectric conversion layer 3 of this solar cell has a bulk heterojunction structure combining a p-type organic semiconductor 15 and an n-type organic semiconductor 16 that is dispersed at the nanometer level and arranged between a transparent electrode 1 and a metal electrode 2. This type of layer configuration is formed by laminating on the surface of a transparent substrate 12.
A vapor deposition method using a low molecular weight material or a coating method using a high molecular weight material is primarily used to form the photoelectric conversion layer 3. Vapor deposition consists of simultaneously vapor-depositing (co-depositing) two types of materials consisting of a p-type organic semiconductor and n-type organic semiconductor, and is characterized by allowing the formation of a layer by layering multiple thin films having respectively different functions in the manner of the bulk heterojunction structure of FIG. 9. On the other hand, coating consists of applying a soluble donor material (p-type organic semiconductor) or acceptor material (n-type organic semiconductor) by dissolving in a solvent, and is characterized by facilitating uniform dispersion of the p-n junction interface to a greater extent than vapor deposition.
In an organic thin film solar cell employing this structure, an exciton E generated as a result of photoabsorption immediately reaches a p-n interface by diffusion and migration at the nanometer level, charge separation occurs, and electron e is transported to electrode 2 through a carrier path coincidentally connected to an n-type organic semiconductor 16, while a hole h is transported to an electrode 1 on the opposite side through a carrier path a p-type organic semiconductor 15, resulting in the generation of electromotive force. In addition, as a result of photoelectric conversion layer 3 being interposed between a p-type organic semiconductor layer 17 and an n-type organic semiconductor layer 18, carriers generated in photoelectric conversion layer 3 can be efficiently collected by an internal interface formed by the two layers of p-type organic semiconductor layer 17 and n-type organic semiconductor layer 18. Since photoelectric conversion layer 3 plays a neutral role, this structure is also referred to as a pin type. In addition, as a result of inserting a hole transport layer 10 and an electron transport layer 11, selective transport of the carrier, reduction of recombination and further improvement of efficiency can be achieved.
In this manner, the problem of a short exciton diffusion distance can be resolved by forming a photoelectric conversion layer comprising a bulk heterojunction structure combining a p-type organic semiconductor and n-type organic semiconductor.
On the other hand, another important issue is the extent to which the charge-separated carriers are transported to the electrodes without being deactivated (recombined). In a solar cell, work is carried out by repeating a cycle in which generated electrons are allowed to flow to the outside or return to the solar cell and recombine with holes. In other words, in the case there is imbalance in the numbers of electrons and holes collected, the actual amount of work is limited by that having the smaller number of carriers. Here, mobility (μ) is used as a parameter indicating transportability. This indicates drift velocity in the presence of a given electric field, and in the case of defining carrier life as ι and field strength as E, the distance over which a carrier is transported is represented by μιE. Thus, a material having a high mobility has a lower probability of recombination and is able to transport a carrier over a longer distance.
Although low molecular weight organic solar cells using fullerene for the n-type organic semiconductor are known to demonstrate high conversion efficiency, since the electron transport capacity of fullerene exceeds the hole transport capacity of p-type organic semiconductors, an excess of electrons end up being transported resulting in the hole transport capacity limiting the overall generated current. In addition, in low molecular weight organic solar cells primarily using metal phthalocyanine for the p-type organic semiconductor, although metal phthalocyanine is superior in terms of optical absorption and dispersibility with fullerene, due to its low mobility, the number of holes that reach the electrode by being transported through phthalocyanine is lower than the number of electrons that reach the electrode by being transported through fullerene. Consequently, phthalocyanine ends up limiting the generated current resulting in the problem of decreased conversion efficiency.
In recent years, however, various high-mobility materials have been developed during the course of development of organic transistors. For example, oligothiophene and pentacene are typical examples of materials used as p-type organic semiconductors, and allow the obtaining of mobility of about 1 cm2/Vs. Although attempts have been made to apply these materials to organic thin film solar cells, operation has of yet only been confirmed for those of a type in which a p-type organic semiconductor layer and n-type organic semiconductor layer are stacked together to form a p-n junction. In solar cells of this type, since the only region in which the carrier can be effectively extracted is the vicinity of flat p-n junctions, in order to enable these solar cells to operate as organic thin film solar cells having high conversion efficiency, it is necessary to form a bulk heterojunction structure as shown in FIG. 9, in which a p-type organic semiconductor and n-type organic semiconductor are combined by co-deposition, and disperse the surfaces of the p-n junctions within a photoelectric conversion layer. However, since oligothiophene and pentacene are molecules in which cyclic compounds are bound in a linear fashion, they easily aggregate on the surface of a substrate during vapor deposition, thereby making it difficult to uniformly disperse molecules of the p-type organic semiconductor in molecules of the n-type organic semiconductor at the nanometer level, and resulting in the problem of it being difficult to effectively improve conversion efficiency.